The capacity of long-haul communication systems, such as “undersea” or “submarine” systems, has been increasing at a substantial rate. For example, some long-haul optically amplified undersea communication systems are capable of transferring information at speeds of 10 gigabits per second (Gbps) or greater in a single optical channel. In order to maximize the transmission capacity of an optical fiber network, a single fiber carries multiple optical channels in a process known as wavelength division multiplexing. For example, a single optical fiber might carry 64 individual optical signals in separate optical channels at corresponding wavelengths evenly spread in the low loss window of an optical fiber, for example between 1540 and 1564.8 nanometers (i.e., spread in channels on 0.4 nanometer centers).
Long-haul communication systems, however, are particularly susceptible to noise and pulse distortion given the relatively long distances over which the signals must travel (i.e., generally 600 to 12,000 kilometers). The performance of an optical transmission system is typically reported as a Q-factor of the signal. The Q-factor gives the electrical signal-to-noise ratio of the digital signal as it enters the receiver's decision circuit, which equivalently gives the bit error ratio of the signal. (Neal S. Bergano, F. W. Kerfoot, and C. R. Davidson, “Margin Measurements in Optical Amplifier Systems”, Photonics Technology Letters, Vol. 5, No. 3, March 1993). Typically operators and owners of digital transmission systems require the system to operate with bit error ratios lower than 1×10−10, which requires the Q-factor to be larger than 16 dB.
Polarization mode dispersion (or PMD) is a differential time of flight for different polarizations through an optical path such as a single-mode fiber. PMD can degrade the average performance of an optical transmission system, and can cause the performance to fluctuate with time. One of the deleterious manifestations of PMD is a degraded waveform or distortion that can change with time. Polarization dependent loss (or PDL) is a differential attenuation for different polarizations through an optical path, such as in an optical component. PDL can also degrade the average performance of an optical transmission system, and can cause the performance to fluctuate with time. One of the deleterious manifestations of PDL is a degraded signal-to-noise ratio that can change with time. Fluctuations in the performance caused by PMD and PDL require the system to operate with added margin to ensure satisfactory performance.
Typically, the added penalties caused by PMD and PDL are limited by placing specifications on the maximum, and average values of PMD and PDL in the system. However, because of the reality of modern manufacturing process, with small likelihood it is possible for highly localized polarization anomalies to be present in an optical transmission system. Once this occurs, it is difficult to locate a polarization anomaly in a manufactured system. Depending on the magnitude of the polarization anomaly, it is possible that the system could simply have a small degradation with little or no system impact. In an extreme case, however, the system could be rendered unusable.
Accordingly, there is a need for a system and method for ascertaining the location of a polarization dependent anomaly in optical communication systems.